Add option to ignore child ordering in comparisons

[stringbean]

This adds a new option (IgnoreChildOrder) that causes child nodes to be sorted before any comparisons are performed.

Fixes #2

[codecov-io]

Codecov Report

Merging #4 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master     #4   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           5      7    +2     
  Lines          75    104   +29     
  Branches        7     13    +6     
=====================================
+ Hits           75    104   +29

Impacted Files	Coverage Δ
...pledragon/xml/compare/NormalisedNodeOrdering.scala	100% <100%> (ø)
.../purpledragon/xml/compare/options/DiffOption.scala	100% <100%> (ø)	⬆️
...ain/scala/software/purpledragon/xml/XmlUtils.scala	100% <100%> (ø)
...software/purpledragon/xml/compare/XmlCompare.scala	100% <100%> (ø)	⬆️

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